PSI - Issue 64

Michael Iten et al. / Procedia Structural Integrity 64 (2024) 1642–1648 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1643

2

Keywords: Distributed Fiber-Optic Monitoring; Deep Boreholes; Renewable Energy Infrastructure; Practical Example.

Nomenclature BHE

borehole heat exchangers

DAS DTS DSS ewb EZF

distributed fiber-optic accoustic sensing distributed fiber-optic temperature sensing distributed fiber-optic strain sensing

Energy Water Bern

Forsthaus energy center mTVD meters true vertical depth TRT thermal response test USM lower freshwater molasse

1. Introduction Monitoring the in-situ temperature distribution is key for the characterization of seasonal geothermal energy storage projects. In this paper, a monitoring method by distributed fiber-optic temperature sensing technology for such a project instrumented in 2022 and 2023 in Bern, Switzerland, is presented.

1.1. Seasonal geothermal energy storage “Forsthaus”

At the “Forsthaus” energy centre (EZF), the local utility company and energy provider Energie Wasser Bern (ewb) operates a waste incineration plant, a wood-fired heating plant and a gas- fired heating plant. The “Forsthaus” geothermal energy storage project aims to store waste heat of the incinerator as well as the surplus from a possible summer operation of the heating plants in the lower freshwater molasse (USM) aquifer unit at a depth of 250 to 500 meters below surface (ewb, 2023). For this purpose, hot water is circulated between a heat exchanger and the USM to heat (or cool) the sandstone lenses of the so- called “Gümmenen” strata (see Fig . 1; compare Kellerhals et al., 2000). In total, a “seasonal energy reserve” of 12 to 15 gigawatt hours is expected from the geothermal energy storage (ewb, 2023).

Fig. 1. Schematic of the “Forsthaus” geothermal energy storage (ewb, 2023) .